1. Field of the Invention
This invention relates to a receiving circuit with automatic frequency controlling.
2. Description of the Prior Art
A receiving circuit with automatic frequency controlling is known. In such a prior art receiving circuit, the local oscillation frequency is controlled by the detected frequency error from the demodulation output.
FIG. 6 is a block diagram of a prior art receiving apparatus with automatic frequency controlling.
The prior art receiving apparatus comprises a receiving circuit 109 for receiving a radio wave signal, a frequency converting circuit 101 for frequency-converting an output of the receiving circuit 109 with a local oscillation signal, a local oscillator 102 for generating the local oscillation signal in accordance with a frequency control signal, a demodulator 103 for demodulating an output of the frequency converting circuit 101 and outputting a demodulated signal, a frequency error detection circuit 104 for detecting a frequency error in the output of the demodulator 103, and an automatic frequency control circuit 106 for generating the frequency control signal with a value of the frequency control signal varied in accordance with the detected frequency error.
FIG. 7 is a time chart of a prior art paging system.
In this paging system, a radio wave signal according to the protocol of CCIR (International Radio consultative Committee) RADIO PAGING CODE NO. 1 and a radio wave signal according to the protocol of FLEX are transmitted from the same base station with the same carrier frequency by a time-division duplex format as shown. Data in the radio wave signal according to the protocol of FLEX and the radio wave signal according to the protocol of CCIR RADIO PAGING CODE NO. 1 include first and second data having different first and second data rates respectively, namely, 3200 symbol/sec and 1200 symbol/sec. Therefore, the automatic frequency controlling operation is not simple.